[FFmpeg-user] Audio Channel mapping: 8 channel to 2 channel stream, dropping input channels 3-8

[Dan Flett]

Hi all,

I am struggling to find a way of mapping channels 1 and 2 of the first input
stream of my audio input.

-map_channel does what I want if I know which numerical stream is the audio
- e.g. -map_channel 0.0.0 -map_channel 0.0.1.  However in my video files,
the first audio stream is sometimes 0.0, sometimes it is 0.1.  I want to
script this so that it always takes the first audio stream.  I can't seem to
use the "a" stream identifier in -map_channel to unambiguously denote audio.

I have tried various filters such as channelmap, pan, etc.  No matter what I
do, the filters always seem to detect the 8 channel input stream as a 7.1
audio format (which it isn't - it's 8 discrete, unrelated audio channels -
not surround sound), and always performs a downmix.  I don't want a downmix,
I just want channel 1 and 2 of the input stream to be channel 1 and 2 of the
output stream and all other input channels to be dropped.

Does anyone have a suggestion here?  I want to avoid a 2-step process - i.e.
piping or using a temp file.

Cheers,

Dan